Cultured human lymphocytes will be used to study the assimilation of selenium as selenite and in a form synthesized by erythrocytes and bound to plasma proteins into the seleno-enzyme glutathione peroxidase. A variety of suspected seleno-intermediates will be used in competition experiments to study the uptake of both forms of selenium by the lymphocytes into glutathione peroxidase. These studies will be conducted using a rapid, electrofocusing analytical technique for measuring labeled selenium assimilation into the enzyme. Cultured lymphocytes will also be used as a source for purification of glutathione peroxidase. The purified enzyme will be compared to glutathione peroxidase isolated from erythrocytes. A seleno-compound, possibly selenobiotin, purified from Phycomyces blakesleeanus will be studied as will the possible formation of selenothiamine by Escherichia coli.